Most present-day generation TPE331 engines are equipped with a Single Red Line computer, more commonly referred to as an (SRL). This system provides several features incidental to normal TPE331 operation, and is intended to reduce cockpit workload. In this article I hope to provide a simple explanation of the SRL and explain its main functions.
Single Red Line
Figure 1
Single Red Line means exactly what it says—the SRL provides the cockpit crew with a single exhaust gas temperature (EGT) limit for all operations other than engine starting. This is accomplished by correcting the EGT signal based upon ambient variables. If power to the controller (computer) is interrupted, the system will display compensated EGT. In that case the actual maximum EGT limits must be manually calculated to compensate for changes in ambient variables by reference to a table located in the Airplane Flight Manual (AFM). Typically, these EGT limits are also shown on a placard (see Figure 1) located near the EGT gauge on the instrument panel.
Another example, the Mitsubishi MU-2B incorporates EGT limits in the Indicated Outside Air Temperature (IOAT) gauge (see Figure 2). Both examples show maximum EGT values for a specific IOAT. Other examples include various RPM in addition to OAT. The idea is to present a placard that is easy to use.
What the SRL does and what it doesn’t do
Figure 2
Before we get into the meat of the SRL functions, let’s review a few turbine-technology related principles. The critical or limiting gas temperature in a gas turbine engine occurs at the first-stage turbine (turbine inlet). Therefore, it’s desirable to monitor the turbine temperature at this location, which is also referred to as Turbine Inlet Temperature (TIT), Station 4, or T4. For practical purposes, however, TIT levels are too high to measure with conventional temperature probes.
As a consequence, thermocouples located downstream, past the last turbine stage (third stage) are used for measuring EGT. This location (Airflow Station) is also referred to as T5. This location provides the lowest gas temperature environment while also offering the best guarantee of satisfactory probe reliability over long periods of service life. (Some airplane types make use of Interstage Turbine Temperature (ITT) systems, rather than EGT.)
One major disadvantage of using an EGT indicating system is the lack of a definite fixed relation to the critical temperature, TIT. Specifically, the relationship between EGT and TIT changes as a function of engine inlet temperature, engine inlet pressure, and engine speed, or RPM, all of which vary as functions of altitude and airspeed. The result is a variable EGT limit, which represents a fixed maximum TIT value. A specific EGT limit must be calculated and monitored for each of the above-mentioned variables to avoid exceeding the TIT limit.
Early in the development of small turbine engines, designers resorted to using the worst-case conditions as benchmarks for setting conservative EGT limits. Their objective was to simplify flight deck workload. The intent was admirable, but the conservative EGT limits resulted in lower power potential and reduced efficiency. Later, with the advent of electronic computers, it became practical to provide EGT indications that indirectly relate to TIT under all prevailing operating conditions. These computers have been designed to receive the appropriate electrical signals, compute a TIT-related value (ΔT), and mathematically adjust compensated EGT to obtain a single (fixed) maximum EGT (EGT INDICATED = EGTCOMPENSATED + ΔT). This is what the SRL computer does.
The SRL system also is designed to provide an automatic reduction in TIT for continuous climb/cruise operations at the same fixed SRL (650 degrees C) limit. This feature, visible on the cockpit EGT indicator when RPM is reduced, is intended to provide a reasonable balance between maximum thermodynamic performance at 100 percent RPM, and a more conservative value for continuous operation at less than 100 percent RPM.
Honeywell has always recommended that pilots use a conservative EGT setting when practical. Of course, maximum allowable power should be utilized when needed; however, any time the pilot can safely reduce TIT it will have a positive effect on engine reliability and operating cost. The reduction in TIT is accomplished by either reducing RPM to the climb/cruise value (96-97 percent RPM), or by reducing EGTINDICATED by 20 to 30 degrees below the maximum allowable limit at 100 percent RPM. (With the SRL switch on and a normally functioning SRL system, maximum EGTINDICATED is always 650 degrees C. However, a failed or switched-off SRL system results in an EGTCOMPENSATED indication with a variable maximum limit, which is always less than 650 degrees C. Refer to the abnormal procedure section in the appropriate AFM, for example: Single Red Line System Failure procedures.)
Auto Start
In addition to providing a calculated SRL EGT indication, the system has an auto start feature, which greatly simplifies engine starting procedures and monitoring. The auto start feature is based on RPM-sensitive speed switches, and a turbine-temperature-sensitive fuel enrichment function. The speed switches enable and/or disable start and in-flight ignition, the single red line EGT calculation, the lubrication system vent valve, the fuel/oil heat exchanger, and a change in the cracking pressure of the secondary fuel nozzles. The auto enrich function provides automatic enrichment of the starting/acceleration fuel schedule based on a measurement of EGT. The system targets approximately 695 degrees C EGT, plus or minus 5 degrees C (adjustable to plus or minus 15 degrees C at the SRL box) by providing additional start and acceleration fuel below 695 degrees C.
The SRL does not provide for auto TTL.
A common misconception is that the SRL can do anything, including automatic torque and turbine temperature limiting. Except for the SRL-conditioned EGT signal, a generic TTL (Torque and/or Temperature Limiter), when installed, functions independently from the SRL system. Consequently, the pilot must monitor EGT and torque limits and manually prevent either exceedance by adjusting the power lever as required.
In summary, the SRL controller, which is designed to be used with the exhaust gas temperature measuring system, eliminates the need for pilots to use correction charts in order to determine the maximum allowable operating temperature for a specific ambient condition. The controller provides for an auto start function and controls sequential actions during engine start and acceleration, such as fuel introduction and enrichment, ignition activation and deactivation, starter cutout, temperature conditioning above 80 percent RPM, and other features. In other words, the SRL reduces pilot workload during normal engine starts and flight operations.
For additional questions on the above subject or any other TPE331 engine related issues, please don’t hesitate to contact Rob Erlick, Honeywell Engines Program Pilot, at (480) 399-4007 or Flight Technical Services at [email protected]. As always, procedures for operating any aircraft system are regulated by the specific AFM. Honeywell operational recommendations are strictly advisory in nature.
